Intense research over the last several years has been devoted to develop treatments and cures for retroviral viral infections in humans and in animals, and particularly for acquired immune deficiency syndrome (AIDS) and AIDS related complex (ARC). Today the public realizes the severe health risk posed by the AIDS retrovirus and that the incidence of ARC and AIDS in humans is increasing at an alarming rate. Furthermore, survival beyond 5 years for those who have contracted AIDS remains remote. Further, AIDS patients whose immune systems have been seriously impaired by the infection, suffer from numerous opportunistic infections and proliferative disease including Pneumocystis carninii pneumonia and Kaposi's sarcoma. No cure for AIDS is known and current treatments are largely without adequate proof of efficacy and have numerous untoward side effects. Due to the severity of the disease, and with death the resultant outcome from the disease, fear of AIDS has resulted in social ostracism and discrimination against those having or suspected of having the disease.
The AIDS virus belongs to a general class of viruses known as retroviruses. As a class, many of the known retroviruses are also oncogenic or tumor causing. Indeed the first two human retroviruses discovered, denoted human T-cell leukemia virus type I and type II or HTLV-I and II, were found to cause rare leukemias in humans after infection of T-lymphocytes. The third such human virus to be discovered, HTLV-III, now referred to as HIV, was found to cause cell death after infection of T-lymphocytes and has been identified as the causative agent of acquired immune deficiency syndrome (AIDS) and AIDS related complex (ARC).
Retroviruses are a class of ribonucleic acid (RNA) containing viruses that replicate by using a reverse transcriptase activity to form a strand of complementary DNA (cDNA) from which a double stranded, proviral DNA is produced. This proviral DNA is then incorporated into the chromosomal DNA of the host cell making possible viral replication by transcription of this integrated DNA and translation of viral messenger RNA into proteins. Replication of the virus occurs by synthesis of viral genomic RNA and its assembly with glycosytated and non-glycosylated viral proteins to form new viral particles. Maturation of virions at the cell surface results in the release of infectious virus progeny.
Retroviral proteins are generally synthesized as polyproteins and virus encoded proteases are required to cleave the precursor polyproteins to form the viral enzymes and structural proteins. For example, the gag and gag-pol precursor polyproteins of retroviruses are synthesized as precursors of viral encoded enzymes and non-glycosylated structural proteins. Similarly, the envelope protein of HTV is a 160 kDa highly glycosylated precursor glycoprotein. The envelope proteins are cleaved by a host-cell protease to give a 120 kDa external glycoprotein (gp 120) and a transmembrane glycoprotein (gp 41). The gp 120 protein contains a high affinity binding site that recognizes the CD4 ligand on CD4-positive human T-helper cells, the known receptor for this virus.
The retroviral proteases also show certain commonality by their inhibition by aspartyl protease-specific inhibitors, Iyoko, et al. Nature 329, 654-67. Similarly, amino acid sequencing of the retroviral proteases show they possess sequence homology. Because of their mutual structural and functional characteristics and their obligate function, the aspartyl proteases serve as a potentially interesting therapeutic target for intervention.
The correctly processed envelope glycoproteins of the retroviruses play an important role in the virus life cycle, which also offers a possible target for clinical intervention. The envelope glycoproteins serve a role in both the initial interaction of the virion and the target host-cell and in the subsequent fusion of the viral envelope and host-cell membranes during penetration. Certain esters of castanospermine are useful in interfering with the processing of the viral envelope glycoproteins and thereby in preventing the initial virus-host cell interaction and subsequent fusion.
The applicants have discovered that the combination of aspartyl protease-specific inhibitors (formula II) with glycoprotein processing inhibitors such as castanospermine derivatives (formula I) result in significant improvements in the inhibition of the HIV virus that would not be expected.